This study showed that 20x and 15x diluted urine could provide enough nutrients and support microalgal growth in open ponds. This pilot project also revealed that it is feasible to grow microalgae in colder climate such as Nordic countries have.
S. acuminatus was able to grow on 20x diluted urine as a batch system for 94 days without any addition of extra urine, trace elements, artificial light or CO2 supply. The result of microalgal cultivation in the pond was comparable to the result of microalgal cultivation in the standard media, OD660 6.8 and 6.4 respectively, and the highest biomass yield was 2.31 g VSS/ l of pond volume. Higher ambient temperatures (20°C), as well as long day-light (19 h/ day), enhanced microalgal growth, and with the reduction of these two exter-nal factors in combination with nutrient depletion, also the microalgal growth was re-duced. Furthermore, this study showed that it is possible to keep a semi-continuous growth of S. acuminatus cultivated on diluted urine (15x and 20x) in open ponds for more than 94 days (maximum OD660 for 15x dilution was 0.8). S. acuminatus showed an ability to remove phosphorus and nitrogen as well as organic matter from the diluted urine. Nev-ertheless, S. acuminatus grown in open ponds was influenced by many external factors such as temperature, light intensity or nutrient composition of the cultivation medium. To describe more precisely how the combination of these factors affected the behavior of S.
acuminatus, more analysis, and more data are needed.
Ideas for improvement of the microalgal cultivation in open ponds and nutrient recovery could include testing higher urine dilutions, using more efficient harvesting methods and potential re-use of the effluent from the harvesting for re-feeding the pond instead of using tap water. Lastly, plans could focus on further microalgal biomass use to close the loop of the circular economy.
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